A fast-moving grass fire burned at least 900 acres Sunday night along Highway 101 between Willits and Ukiah, forcing evacuations near Redwood Valley and slowing traffic on the major highway artery.

The blaze, known as the Grade fire, led to evacuations in the Baker Creek subdivision in Redwood Valley and the valley floor north of Redwood Valley, according to the Cal Fire website. The subdivision is south and east of the fire off Baker Creek Road.

A Cal Fire spokeswoman said 450 personnel were battling the fire on steep, mountainous terrain of grass, brush and oak woodland. She said 21 Cal Fire engines, seven engines from local governments, six water tenders and eight Cal Fire bulldozers, plus helicopters and aircraft were involved.

“They’re making good progress” on the fire, the spokeswoman, Tricia Austin, said at 11:30 p.m.

Cal Fire’s next briefing on the fire will be at 7 a.m. Monday.

The fire began around 2:50 p.m. on a long incline of the highway roughly 20 miles north of Ukiah. At 9 p.m. the fire was just 10 percent contained.

The CHP’s Ukiah traffic incident website suggested the blaze may have started as a car and trailer fire. The site reported the fire spread to grass on the adjacent hillside within eight minutes of the first report of flames. It also recorded the challenges of firefighters seeking to reach the blaze on a crowded highway with a concrete barrier separating the north and southbound lanes along much of the grade.

Austin said Sunday evening both northbound and southbound lanes remained open, but northbound traffic was slowed.

Jim Crowhurst, a former Press Democrat employee who resides in Redwood Valley, said the fire was burning in steep terrain within 10 miles of his home.

“I can see the smoke from where I live,” he said.

Crowhurst said he spotted two firefighting aircraft in the skies overhead about 8:15 p.m. Earlier he had spied an even larger plane, which other reports suggested may be Cal Fire’s DC-10 air tanker, capable of holding 50 tons of water or retardant.

You can reach Staff Writer Robert Digitale at 707-521-5285 or robert.digitale@pressdemocrat.com. On Twitter @rdigit.